The accuracy of physically based lighting simulation and realistic rendering is considered in the paper. One of the main global illumination simulation methods is forward Monte Carlo ray tracing. The analysis of global illumination simulation errors (discretization and stochastic ones) is presented. The quality of rendering is determined mainly by stochastic error. Also the issue of prediction of calculation time necessary to obtain requested simulation accuracy is discussed and practical approach to solve the issue is suggested. The prediction mechanism with rich user interface and criteria for simulation process termination is presented.